Which of the following is most acidic
(i) Butane (ii) But-1-ene (iii) But-1-yne (iv) But-2-yne ?

Answer

To determine which of the given hydrocarbons is the most acidic, we need to analyze the acidity of each compound based on their hybridization and the ability to release a proton (H⁺).

### Step-by-Step Solution:

1. **Understanding Acidity**:
- Acidity is defined by the ability of a compound to release H⁺ ions. The more easily a compound can release H⁺, the more acidic it is.

2. **Identifying the Compounds**:
- We have four compounds to consider:
- (i) Butane (C₄H₁₀)
- (ii) But-1-ene (C₄H₈)
- (iii) But-1-yne (C₄H₆)
- (iv) But-2-yne (C₄H₆)

3. **Determining Hybridization**:
- **Butane**:
- All carbon atoms are sp³ hybridized (single bonds).
- S character = 25% (4 hybrid orbitals).
- **But-1-ene**:
- One carbon is sp² hybridized (double bond), and the others are sp³.
- S character = 33.33% (1 sp² carbon).
- **But-1-yne** and **But-2-yne**:
- Both have at least one carbon that is sp hybridized (triple bond).
- S character = 50% (2 sp carbons).

4. **Comparing Acidity Based on S Character**:
- The acidity increases with the increase in s character:
- Butane (25% s) < But-1-ene (33.33% s) < But-1-yne (50% s) = But-2-yne (50% s).
- But-1-yne and But-2-yne both have the highest s character.

5. **Identifying Terminal vs. Internal Alkynes**:
- But-1-yne is a terminal alkyne, meaning it has a hydrogen atom attached to the carbon with the triple bond, allowing it to release H⁺ easily.
- But-2-yne is an internal alkyne and does not have a hydrogen attached to the carbon with the triple bond, making it less acidic.

6. **Conclusion**:
- Since But-1-yne can release H⁺ more easily than But-2-yne due to its terminal position, it is the most acidic among the given options.

### Final Answer:
- The most acidic compound is **(iii) But-1-yne**.

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